Surge is an unwanted phenomenon in centrifugal compressors which occurs when the fluid flow rate through the compressor is reduced, to an unstable level. When the flow rate is reduced to a point below a required minimum flow rate, fluid resistance increases at the compressor discharge port and as the discharge pressure increases until surge occurs. During the occurrence of surge, the direction of fluid flow is reversed as the higher pressure fluid at the discharge flows backward into the compressor.
Surge is undesirable for a number of reasons. Compressor surge produces unstable fluid flow within the compressor, increased thrust loads on compressor components, produces loud noises, and increases the amount of heat generated within the compressor. Frequently, one of the consequences of surge is damage to compressor components.
One conventional way of avoiding surge is by increasing the fluid flow rate through the compressor, often by recirculating fluid back through the compressor. Although surge is avoided by increasing the flow rate through the compressor, such recirculation of flow through the compressor adversely affects the compressor efficiency and, therefore, the cost of operation.
Surge in centrifugal compressors can be understood to occur at low flow conditions below which the rotating impeller cannot impart sufficient momentum to the flow to overcome the suction to discharge head. During surge, flow through a compressor becomes unstable and momentarily reverses direction, thereby shock loading the compressor, disrupting system operations, and potentially damaging the compressor. Centrifugal compressors that operate over a range of conditions must be protected from inadvertent entry into surge.
The approach of a compressor to surge is normally monitored by external measurement of flow rate and pressure to determine the compressor's condition relative to a line of flow and head that is selected as the surge control limit. This common approach is inferential and is dependent on proper selection of the surge control line. Currently, no widely accepted reliable methods are available to establish an accurate surge margin line. Setting the surge limit at too high a flow rate limits the efficient operation of the compressor and results in wasted energy and an unnecessary loss of efficiency during low flow conditions. Setting the surge limit too low can result in the compressor reaching surge and being damaged.